The present invention relates to communications systems and methods, and more particularly, to multicarrier communications systems and methods.
As demands for very high-speed communications applications, such as Internet applications, have increased, there has been a concomitant increase in demand for high-speed communication services that have relatively low installation and service charges. To meet those demands, various Digital Subscriber Line variants (xDSL) that use ordinary copper telephone lines installed in existing houses and offices have been offered and/or proposed. xDSL services include high data-rate DSL (HDSL) substituted for the existing T1 line, symmetric DSL (SDSL) substituted for T1 or E1 using one twisted-pair copper line, and asymmetric DSL (ADSL) capable of transmitting large-capacity data in a public switched telephone network (PSTN) environment.
ADSL is “asymmetric” in that greater bandwidth and larger amounts of data are transmitted downstream from a central office (CO) to a remote terminal (RT) than from the remote terminal to a central office. In typical ADSL applications, which often use existing telephone lines, it is possible to communicate data at a high speed and plain old telephone service (POTS) at the same time. A typical transmission rate for ADSL is 8 Mbps in the downstream direction and 640 kbps in the upstream direction.
Unfortunately, however, a modulated signal, such as a DSL carrier, can transfer electromagnetic energy to adjacent copper lines disposed within the same cable bundle. A cross coupling of such electromagnetic energy is called “crosstalk.” In a typical telephone network, a pair of insulated copper lines is bound with what is referred to as a “cable binder.” Adjacent channels that are carried on conductors disposed within the same cable binder and which transmit and/or receive information within the same frequency range often generate significant crosstalk. As a result, received signals may be somewhat altered from originally transmitted waveforms.
Such crosstalk can be classified into two types, near end crosstalk (NEXT) and far end crosstalk (FEXT). NEXT typically is more significant because high energy signals generated from a transmitter at one end of a cable can causes very large crosstalk in a signal received by a receiver coupled to the same end of the cable. In contrast, FEXT is generated by equipment disposed at the opposite terminal of the cable, and is generally smaller than the NEXT because the copper line typically attenuates remotely generated interference.
A Time Compression Multiplexing Integrated Service Digital Network (TCM-ISDN) repeatedly performs upstream and downstream transmissions of data during periods of a so-called TCM timing reference (TTR). During a first half period of the TTR, the ISDN central office transmits data to the ISDN remote terminal. During the second half period of the TTR, the ISDN remote terminal transmits data to the ISDN central office. In TCM-ISDN, sources of NEXT and FEXT noises are often referred to as “TCM-ISDN interference.”
In order to communicate in a TCM-ISDN environment, an ADSL transceiver typically transmits a large amount of data during periods when FEXT is the main source of interference and transmits a relatively small amount of data or no data during periods when NEXT is the main source of interference. One technique for reducing NEXT interference is called the dual bit-map (DBM) technique, wherein different bit-maps are used in respective FEXT and NEXT periods. Another technique is called the single bit-map (SBM) technique, wherein data is transmitted only during FEXT-dominant periods. Yet another technique is the so-called SNR technique that selectively uses DBM and SBM techniques based on signal to noise ratio measurements.
In applications where an ADSL service is provided in a TCM-ISDN environment, accurate network time synchronizations can be performed between the ADSL service and the TCM-ISDN service. Additionally, in case where data is transmitted using the DBM method, a transmission rate can be maximized. In the ADSL system under the environment of the TCM-ISDN, one frame is 250*(68/69) μs in length, i.e., about 246 μs. One frame is provided with a series of 345 frames and is 250*(68/69)*345 μS in length, i.e., 85 ms.
Among supplementary provisions of the ADSL modem standard, ADSL Annex C requires that the receiving side (i.e., the remote terminal) should adopt hyperframe synchronization with respect to a sending side (i.e., the central office) at the beginning of an initialization process. This hyperframe synchronization is typically carried out after frame synchronization is achieved. According to the ADSL Annex C, the central office simultaneously transmits a pilot tone of 276 kHz and a TTR indication tone of 207 kHz during the initialization process. It is determined whether a received frame is a FEXT frame or a NEXT frame from the phase of the indication tone. Accordingly, after frame synchronization is achieved and the remote terminal determines whether a received frame is a NEXT frame or a FEXT frame by analyzing the phase of the TTR indication tone, it is possible to determine a sequence number of the received frame in the hyperframe.